Ultraviolet absorbing copolymers

ABSTRACT

Photostable and weather stable absorping copolymers have been prepared from acrylic esters such as methyl methacrylate containing 0.1 to 5% of an 2-hydroxy-allyl benzophenone, preferably the 4,4&#39; dimethoxy derivative thereof. The pendant benzophenone chromophores protect the acrylic backbone and when photoexcited do not degrade the ester side chain, nor abstract hydrogen from the backbone.

DESCRIPTION Technical Field

The present invention relates to ultraviolet absorbing polymers,monomers therefor and, more particularly, to allyl hydroxy-benzophenonesand to acrylic copolymers thereof.

It has been attempted to protect photodegradable polymers byincorporating various absorbers or stabilizers into the film, either ascompounded additives dispersed in a film or as copolymerizable monomers.Additives are not weather stable. When the additive containing films areexposed for long periods to the environment, these fugitive agentsmigrate to the surface of the film and vaporize.

Benzophenones are a widely utilized class of U.V. screening agents.Vinyl derivatives of benzophenones have been copolymerized withhydrocarbon alpha-olefins such as styrene, ethylene or propylene.However, styrene polymers are intrinsically unstable to ultravioletradiation and so much of the U.V. absorbing comonomer must be added thatthe mechanical properties of the copolymer are adversely affected.Furthermore, the benzophenone comonomers exhibit a very broad absorptionband extending into the visible range reducing transparency andefficiency of underlying devices such as photovoltaic solar cells.

Copolymers of commercially available U.V. absorbing monomer: ##STR1##show poor absorption in the 300 to 350 nm range and a very broadabsorption edge trailing into the visible at frequencies above about 400nm. Acrylic ester polymers show are inherently stable to ultravioletradiation so that a copolymer with a very small amount of 2-hydroxybenzophenone could be capable of forming thin U.V. screening films.However, the excited benzophenone moiety might readily transfer energyto the ester side chain since the triplet states of both are probablyclose enough in energy to permit endothermic energy transfer. Excitationof the acrylic ester side chain leads to C--O bond cleavage.Furthermore, the excited chromophore could also result in abstraction ofhydrogen from the backbone. The excited chromophore may react withoxygen leading to photooxidation and consumption of the chromophore.

STATEMENT OF THE INVENTION

A copolymer of certain allyl benzophenones, addition copolymerizablewith acrylic esters has been developed in accordance with thisinvention. The resulting copolymer has chemically bound chromophoreswhich cannot be removed through physical processes. The acrylicbackboned is protected by benzophenone moieties which when excited donot degrade the ester side chain nor abstract hydrogen from thebackbone.

The copolymers can be cast into a thin, hard, weatherable, transparentfilm which exhibits strong absorption throughout the ultraviolet lightrange (295-400 mm) present in sunlight at ground level and a sharp slopeat the visible end of the range. A 0.02 cm thick film of copolymercontaining about 0.5% of the chromophore can achieve 99% attenuation of360 mm radiation, but is essentially identical to the homopolymeracrylate in terms of structure and reactivity. Photodegradationexperiments show the copolymer to be extremely photostable showing noindication of photooxidation, chain scission or photocrosslinking. Along term extraction in methanol showed no change in U.V. screeningability. The copolymers have high glass transition temperatures andmolecular weights above 30,000 so that at room temperature, they arehard and dust resistant.

The copolymer of the invention can be applied as a film to protectsubstrates that are photodegraded by ground level solar radiation suchas photovoltaic solar cells, solar reflectors, polyethylene orpolypropylene pipes elastomers and the like. The films can be depositedon the surface of the substrates by extrusion, solvent casting orlamination.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is spectral response of o-nitrobenzaldehyde actinometer;

FIG. 2 is rate of change of absorptance vs. wavelength for copolymers ofExamples 6 and 7; and

FIG. 3 is change of electronic spectra of copolymer films as a functionof irradiation period.

DETAILED DESCRIPTION OF THE INVENTION

The copolymers of the invention comprises a copolymer of at least oneacrylate ester of the formula: ##STR2## where R¹ is hydrogen or alkyl of1 to 6 carbon atoms and R² is alkyl of 1 to 6 carbon atoms with 0.1 to5% by weight of an allyl, 2-hydroxy benzophenone of the formula:##STR3## where Z is --(CH₂)_(m) --CH═CH₂ wherein m is 1 or 2, but atleast one of p and q is 1; R³, R⁴, R⁵, R^(3'), R^(4') and R^(5') areindividually selected from H, OR⁶ or Z where R⁶ is methyl, ethyl orpropyl and at least one and no more than two of said R³, R⁴, R⁵, R^(3'),R^(4'), R^(5') is Z. Preferably R³ is Z, R^(3'), R⁵ and R^(5') are H andat least one of R⁴ and R^(4') are alkoxy.

The closer spacing of the allyl group to the ring nucleus provides amore rigid, restricted linkage of the benzophenone to the acrylatebackbone having less degree of freedom of movement. This providesimproved absorption in the 300-350 nm range. The addition of methoxygroups, especially when in the position alpha to the allyl groupprovides a much sharper absorption edge at the higher frequencies andless trailing into the visible.

The polymer can be formulated with other compatible additioncopolymerizable monomers such as diacrylates or triacrylates. Thecopolymer is formed by free radical initiated and catalyzed additionpolymerization. The free radicals can be generated by heat, radiation orfree radical catalysts such as peroxides, peroxy compounds or azocompounds. Since the benzophenone acts as a chain stoppingpolymerization inhibitor, the minimum concentration of initiator shouldbe at least 0.5 part per hundred parts of total monomers, usually from 1to 5 parts.

Copolymers were prepared by reaction of an excess of acrylic ester suchas methyl methacrylate at a ratio of at least 5/1, preferably at least10/1 with the allyl benzophenone in polar solvent, such as dioxanedimethyl sulfoxide (DMSO), dimethyl formamide (DMF) or their mixtures.The amount of monomers can be from 1 to 10% of the solution. Thepolymerization reaction is preferably conducted at an elevatedtemperature of from 35 degrees C. to 100 degrees C. followed by recoveryand extraction of the polymer. Examples of practice follow:

EXAMPLE 1 Synthesis of 2,4-Dihydroxy-4'-Methoxy Benzophenone

Aluminum chloride (48 gm, 0.36 moles) was added gradually, with stirringto a mixture of 31 grams (0.18 moles) of anisoyl chloride, 19.82 grams(0.18 moles) of resorcinol and 300 ml. of dry nitrobenzene during 60minutes. Sixty hours later the reaction mixture was poured on 200 gramsof ice and 100 ml. of concentrated hydrochloric acid. After removal ofthe nitrobenzene with steam the residual crystalline solid product waswashed with water and collected. The cake was then washed withd cold(-50 degrees C.) methanol to remove tarry impurities and dried.

Yield: 37 grams (83.4% based on anisoyl chloride); M.P.-154-156 degreesC.; thin-layer chromotography on 13181-Silica gel in acetone showedessentially pure product.

EXAMPLE 2 Synthesis of 4,4'-Dimethoxy-2-Hydroxy Benzophene.

Methyl iodide (10.6 ml, 0.17 moles) was added portion-wise during aperiod of one hour to a solution of 37 grams (0.151 moles) of2,4-dihydroxy-4'-methoxy benzophenone in 151 ml boiling acetone,containing 62 grams (0.46 moles) of potassium carbonate. All startingmaterials were dried prior to use and the reaction was conducted underanhydrous conditions. After the addition was finished, the reactionmixture was heated under reflux for an additional 45 minutes. Theproduct was isolated by pouring the mixture into 600 ml of water andfiltration. The final product was washed several times with water anddried under vacuum.

Yield: 37 grams (95% of theory); M.P.-114-116 degrees C. The productgave an intense red-brown ferric chloride reaction in alcohol and washomogenous on TLC.

EXAMPLE 3 Synthesis of 2-Allyloxy-4,4-Dimethoxy Benzophenone

A mixture of 37 grams (0.143 moles) of 2-hydroxy-4,4'-dimethoxybenzophenone, 13.85 ml. (0.16 moles) of allyl bromide, 59 grams (0.429moles) of dry, powdered potassium carbonate and sufficient dry acetone(˜150 cc) to give an easily stirred mass was stirred and refluxed forsix hours. Water (200 ml) was then added and the resultant oily mixturewas extracted with three 75 ml portions of methylene chloride.

The combined organic phase extracts were washed with two 75 ml portionsof 10% sodium hydroxide solution, twice with water, dried over anhydrousmagnesium sulfate and distilled under reduced pressure to oil. Theproduct gives a negative ferric chloride test, indicating absence ofunreacted phenol and that etherification has occurred.

Yield: 38.3 grams (90% of theory) homogenous on TLC.

EXAMPLE 4 Synthesis of 2-Allyloxy-4-Methoxy-Benzophenone

The ether was prepared as in the preceding example from 100 grams (0.45moles) benzophenone, 61 grams (0.45 moles) K₂ CO₃, 2.5 grams (0.45moles) allyl bromide and 200 ml of acetone. The product (pale yellowoil) was isolated after 8 hours reflux and was rearranged withoutfurther purification.

Yield: 81 grams (69% of theory) Homogenous on TLC; negative ferricchloride test.

EXAMPLE 5 SYNTHESIS OF 3-Allyl-4,4'-Dimethoxy-2-Hydroxy-Benzophenone

Thirty-seven grams of 2-allyloxy-4,4'-dimethoxy-benzophenone were placedin thin-walled pyrex tube, equipped with air-condenser, magneticstirring, nitrogen inlet and thermometer. With stirring and undernitrogen, the product in the tube was placed in an oil-bath at 240degrees C. After two minutes a sudden temperature surge to 295 degreesC. occurred and heating was discontinued. The reaction mixture wascooled to room temperature and an equal volume of methanol was added.The mixture was cooled to -20 degrees C., upon which yellow crystallineproduct was obtained. The product was recrystallized frommethanol/acetone, yielding homogenous product of TLC (10% acetone inpetroleum ether). The material gives a positive test with ferricchloride indicating that rearrangement has occurred.

Yield: 22 grams (60% of theory); M.P.=119-120 degrees; C. calculated forC₁₈ H₁₈ O₄ (298.32); C, 72.46%; Found: C, 72.56%; H, 6.26%.

The ultraviolet spectrum of the product exhibits λ max at 300 nm with anextinction coefficient of 20,855 in ethyl alcohol.

EXAMPLE 6 METHYL METHACRYLATE3-Allyl-4,4'-Dimethoxy-2-Hydroxy-Benzophenone Copolymer

A 250 ml., three-necked flask equipped with nitrogen inlet, condender,thermometer, and a magnetic stirrer was charged with a solution of 0.5grams of 3-allyl-4,4'-dimethoxy-2-hydroxy-benzophenone, 5 grams offreshly distilled methyl methacrylate and 0.1 grams ofα,α'-azobisisobutyronitrile in 100 ml of dimethyl sulfoxide/dioxane(1:9). All starting materials and solvents were dried prior to use. Thereaction vessel was flushed with dry nitrogen for 10-15 minutes and thenplaced in a constant temperature water bath at 55 degrees C. for 12hours, then was raised to 80 degrees C., and maintained at thattemperature for an additional 24 hours. The contents of the flask werethen slowly poured into a waring blender, containing 500 ml of methanol.The product was collected by filtration, washed with two-50 ml portionsof methanol, and dried in vacuo at 40 degrees C. The crude polymer wasdissolved in methylene chloride, the solution was filtered and thefiltrate added to a large excess of methanol as described above. Thepolymer was further purified by exhaustive extraction with methanol in asoxhlet extraction apparatus for 100 hours. Small samples of the polymerwere taken out, dissolved in methylene chloride and their UV-spectrumtaken. The polymer extraction continued until a constant UV-absorbancewas obtained at 320 mm. The product contains 0.43% of the2-hydroxy-benzophenone UV-absorber as copolymer, and no other change inthis concentration was observed after the 40th hour of extraction. Thefinal product was dried in vacuo at 40 degrees C. to yield 4.95 grams ofcopolymer. The gel-permeation chromatography curve indicates thathomogenous copolymer with M_(w) =83,000 has been produced. A Watersassociates, Model 6000 gel permeation chromatograph, packed with a bankof four styragel columns and standaridized with commercial monodispersepolystyrene fractions was used to obtain the GPC curves. The ultravioletspectrum of the material exhibits λ max at 304 nm (a=0.264) in methylenechloride.

EXAMPLE 7 Methyl Methacrylate3-Allyl-4,4'-Dimethoxy-2-Hydroxy-Benzophenone Copolymer

The procedure of Example 6 was repeated substituting Permasorb MA forthe allyl-hydroxy-benzophenone. A copolymer was produced.

A solution of the copolymer of Example 6 in CH₂ Cl₂ was analyzed on aHPLC using four capillary columns packed with "μ-styragel". Therefractive index peak matched with the UV (250 nm) peak indicating thatthe chains containing the chromophores had the same approximatedistribution as the chains with no chromophores. However, the UV peakhave a lower apparent molecular weight distribution than the refractiveindex peak indicating that the chromophore bearing chains had a lowermolecular weight distribution. By following these two distributionssimultaneously, it is possible to identify the site of photodegradationin the copolymer. Since incorporation is only about 0.5%, theprobability of one chain containing more than one chromophore is quitesmall assuming random copolymerization. Hence, the copolymer is almostidentical to PMMA in terms of structure and reactivity and is notseparated from it either by extraction or by passage through HPLC. It isnot expected that blocks of allyl benzophenone will form since it isexpected that a chain radical ending in allyl benzophenone will decaymainly through recombination, probably with a chain radical with methylmethacrylate at the end (R·₂) or with the initiator radical (P·), sinceconcentration of R₂ · or P· are expected to be much higher than R₁ ·. Ina sense, the allyl benzophenone acts as a polymerization inhibitormaking the minimum concentration of initiator needed very high. It isexpected that much of the low molecular weight product containing highconcentration of initiator end groups (which may be potentiallyphotolabile) is removed through extraction. The copolymer was dissolvedin CH₂ Cl₂ and cast into optically clear films which were dried toremove solvent. These films were irradiated in a merry-go-round typeapparatus with light from a medium pressure Hg (Hanovia) lamp filteredthrough 8 mm of pyrex, which attenuates 99% of the light incident on itat 297 nm. The spectral distribution of this irradiance has beenmeasured and consists of peaks at 302, 313, 366 nm in the ultravioletregion which are absorbed by the sample. Since the objective was toevaluate the photo stability of these films in sunlight at ground level(Airmass one), it was decided to use a broadband ultraviolet source asdescribed above without further wavelength selection. Test films of twothickness were used, 4×10-³ cm and 2×10-³ cm approximately. The lightflux absorbed by the sample integrated in time was estimated by using anactinometer film containing o-nitrobenzaldehyde which has approximatelythe same wavelength response as the test films (FIG. 1). Light fluxabsorbed by the films was calculated by choosing an actinometer film ofapproximately equal wavelength response and calculating conversion inmilligrams of o-nitrobenzaldehyde to o-nitrosobenzoic acid per second.If this is a, then number of photons absorbed by the test film persecond is (a/251)×10-³ ×2.0×6×10²³, when 151 is the molecular weight of0-nitrobenzaldehyde, 2.0 is the reciprocal of quantum yield ofconversion in the actiometer and 6.0×10²³ is Avogardo's number. Thisvalue was compared with energy of radiation absorbed per unit timeobtained from spectroradiometric measurements on the lamp and absorbanceplot of the actinometer film. Table 1 gives the number of photonsabsorbed by the actinometer film estimated radiometrically and compareswith the experimental quantity. In this table, the number of photonsabsorbed by the copolymer films is also included.

    ______________________________________                                                                  No. of photons                                      Film Thickness                                                                            Time of       Absorbed (× 10.sup.20)                        (× 10.sup.-3 cm)                                                                    Exposure (hrs)                                                                              per cm.sup.2                                        ______________________________________                                        2           0             --                                                  4           7             21.7                                                4           16            49.6                                                2           77            215.6                                               2           135           338                                                 4           448           1388.8                                              ______________________________________                                    

The following table gives the total dosages and times of irradiation ofthe films.

    ______________________________________                                                             Film        Film                                                              (d = 4 × 10.sup.-3                                                                  (d = 2 × 10.sup.-3                               Acintometer                                                                              cm)         cm)                                                    Absorption Absorption  Absorption                                   Wavelength                                                                              Photons/   Photons/    Photons/                                     range     cm.sup.2 sec                                                                             cm.sup.2 sec                                                                              cm.sup.2 sec                                 ______________________________________                                        ≦293 nm                                                                          2.4 × 10.sup.13                                                                    9.1 × 10.sup.13                                                                     8.9 × 10.sup.13                        293-299.6 8.9 × 10.sup.13                                                                    3.3 × 10.sup.14                                                                     3.3 × 10.sup.14                        299.6-307.8                                                                             1.0 × 10.sup.15                                                                    4.0 × 10.sup.15                                                                     3.9 × 10.sup.15                        307.8-323.5                                                                             7.0 × 10.sup.14                                                                    2.8 × 10.sup.15                                                                     2.6 × 10.sup.15                        323.5-350 1.7 × 10.sup.16                                                                    6.3 × 10.sup.16                                                                     6.1 × 10.sup.16                        350-385.3 5.0 × 10.sup.15                                                                    1.9 × 10.sup.15                                                                     1.2 × 10.sup.16                        Total                                                                         Absorption                                                                              2.38 × 10.sup.16                                                                   8.9 × 10.sup.16                                                                     7.9 × 10.sup.16                        Overall                                                                       Absorption                                                                    from Chemical                                                                 Conversion                                                                              2.2 × 10.sup.16                                               ______________________________________                                    

In terms of AM-1 sunlight, the largest period of exposure corresponds toan outdoor exposure of 1.5 yrs. The films were monitored by UV-visiblespectroscopy and FT-IR spectroscopy as well as HPLC equipped withcapillary columns packed with μ-styragel. A preliminary experimentindicated negligible weight loss on irradiation.

FIG. 2 shows a plot of dε/dλ vs wavelength for the copolymer of Examples6 and 7. dε/dλ is taken as screening efficiency since it is a measure ofeffectiveness of screening at short wavelength (300-360 nm) withoutimparting color to the film or absorbance at wavelengths greater than400 nm. FIG. 3 is a plot of A vs wavelength when A=OD.sub.λ^(t) /OD₈₀^(o) when t is time of exposure. This plot indicates that there isnegligible change in structure of the chromophore due tophotodegradation at the two film thicknesses studied. FT-IR spectrayields the same results, in other words, spectroscopic results indicatelittle change in the structure of the chromophore. A plot of differencehydroxyl absorption at 3580 cm-¹ vs time of exposure was prepared. Thehydroxyl band is broad and cannot be precisely analyzed. Theconcentration is quite small, but it would still be sufficient to causea measurable change in the electronic absorption spectrum if itrepresented consumption of the chromophore. Hence, it is assigned to theMMA segments of the polymer and a slow process of photo oxidationcatalyzed by the chromophore cannot be ruled out. Methanol is ruled outsince no methanol is found in HPLC analysis performed on the same films.Absence of methanol also excludes excitation transfer to the side chain(ester). Table 3 gives molecular weight distributions calculated fromboth refraction index and UV peaks.

Table 3. Molecular weight distributions as function of exposure period.

    ______________________________________                                                  Time of                                                             Detection Mode                                                                          Exposure (hr)                                                                             --M.sub.n                                                                              --M.sub.w                                                                            --M.sub.w /--M.sub.n                    ______________________________________                                        RI        0           46000    87000  1.89                                    UV        0           31000    69000  2.21                                    RI        448         42000    77000  1.83                                    UV        448         34000    79000  2.36                                    ______________________________________                                    

Quantum yield of chain scission calculated from refraction indexmeasurements yield a value of 1.8×10-⁸ for quantum yield of chainscission, assuming that M_(n).sbsb.t /M_(n).sbsb. -1) C_(s) =φc-s-xP_(t) when φc-s is quantum yield of chain scission, P_(t) is the numberof moles of photons absorbed in time and Cs is the number of moles ofchains present in the films.

This exceedingly small value is an indication of the extremephotostability of the polymer. There is a simultaneous increase of themolecular weight of the chains bearing chromophores which generate theUV signal. A simple calculation indicates that the probability of achain containing one or more chromophores in this system is about 66%,assuming random copolymerization. Hence, using the UV data of Table 2 itis possible to calculate the quantum yield of branching here for thechains bearing chromophore units. This quantum yield is approximately1.6×10⁻⁸. Consideration of the data that the electronic absorptionspectrum remains constant (to within 2%) and the chromophore bearingchains undergo branching while the copolymer as a whole undergoes a netdecrease in molecular weight. A photochemical mechanism has beendeveloped which identifies the tertiary hydrogens on the chromophorebearing chains as the primary site of attack. The total concentration oftertiary hydrogens in the copolymer per sq cm is about 4.2×10⁻⁸ moles in1 sq cm of film. After 448 hours of irradiation the number of moles ofbranch points generated is about 4.8×10⁻⁹ moles/sq cm, and the netnumber of chain scissions is about 6×10⁻⁹ moles/sq cm of the film.

Transient absorption spectra were taken after various time delaysfollowing flash photolysis excitation with a 355 nm pulse. The lifetimeof the shortlived transient is estimated to be ˜10 ps, while thelifetime of the long-lived transient is estimated to be >400 ps. Theshort-lived transient is presumably the first singlet, while thelong-lived transient is the first triplet. The triplet lifetime isextraordinarily short relative to that of triplet benzophenone and isclearly due to the intramolecular hydrogen bonding. This exceedinglyshort triplet lifetime also explains the photostability of this system.Both hydrogen abstraction and energy transfer processes leading todegradation are expected to be activated and hence, far less efficientcompared with the ultra fast radiationless deactivation process whichefficiently depopulates the triplet.

Thus, a stable copolymer containing a 2-hydroxy benzophenone derivativeas a pendant group has been prepared. The stability of the copolymer isclearly due to the short lifetime of the reactive excited states of thechromophore. Copolymers prepared according to this invention are usefulas transparent U.V. screening covering or encapsulating films.Copolymers of improved physical properties can be prepared bysubstitution of a portion of methyl methacrylate with other monomerssuch as n-butyl methacrylate.

It is to be realized that only exemplary and preferred embodiments ofthe invention have been described and that these and many otheralternative, variations, adaptations and modifications are permissablewithout departing from the spirit and scope of the invention as definedin the following claims.

We claim:
 1. An ultraviolet absorbing copolymer of an additionpolymerized copolymer of a monomer mixture comprising: an acrylic esterof the formula: ##STR4## where R¹ is hydrogen, alkyl of 1 to 6 carbonatoms andR² is alkyl of 1 to 6 carbon atoms; and
 0. 1 to 5% of ahydroxy-benzophenone of the formula: ##STR5## where R³, R⁴, R⁵, R^(3'),R^(4'), R^(5') are individually selected from H, OR⁶ or Z where R⁶ ismethyl, ethyl or propyl and Z is --(CH₂)_(m) --CH═CH₂ where m is 1 or 2and at least one but no more than two of R³, R⁴, R⁵, R^(3'), R^(4') orR^(5') are Z.
 2. A copolymer according to claim 1 in which R³ is Z andat least one of R⁴ and R^(4') are OR⁶.
 3. A copolymer according to claim2 in which R^(3'), R⁵ and R^(5') are H, and R⁴ and R^(4') are both OR⁶where R⁶ is methyl and m is
 1. 4. A copolymer according to claim 1 inwhich R¹ and R² are methyl.